Electrically tunable microwave devices based on a YBa.sub.2 Cu.sub.3 O.sub.7-x /SrTiO.sub.3 multilayer have been extensively investigated. In those designs, advantage is taken of the dc electric field tunability of a nonlinear dielectric SrTiO.sub.3 film and the very low conductor losses in high-temperature superconducting YBa.sub.2 Cu.sub.3 O.sub.7-x (YBCO) electrodes at cyrogenic temperatures below 90 K. For practical dc electric-field tunable microwave devices such as voltage-tunable resonators and voltage-tunable filters, it is desirable to grow high-quality dielectric SrTiO.sub.3 thin films that have as large a dielectric tunability and as low dielectric losses as possible. Unfortunately, previous studies have shown that the effective loss tangent from a generic YBCO/SrTiO.sub.3 multilayer is on the order of 10.sup.-2 which is much higher than a value of 10.sup.-4 observed in a single-crystal SrTiO.sub.3.
Others have made efforts to look for new nonlinear dielectric materials and to use different dopants such as calcium and/or zinc in SrTiO.sub.3 to reduce the loss tangent.
It has been shown that dielectric losses in SrTiO.sub.3 film play the most important role in determining the performance of microwave devices. These dielectric losses include losses in the bulk of the SrTiO.sub.3 film, the losses at the interface between the substrate and the SrTiO.sub.3 film, and the losses at the interface between the SrTiO.sub.3 film and the YBCO electrode.
Thus, one object of the present invention is to improve the microstructural properties of SrTiO.sub.3 films so as to enhance the microwave properties of devices based on a YBCO/SrTiO.sub.3 multilayer.
Another object of the present invention is to provide a thin homoepitaxial LaAlO.sub.3, interlayer between a LaAlO.sub.3 substrate and a SrTiO.sub.3 film to reduce the defect density in the SrTiO.sub.3 film.
Still another object of the present invention is to provide a coplanar waveguide device structure including SrTiO.sub.3 as a nonlinear dielectric and superconducting YBCO as an electrode.
Yet another object of the present invention is to use a thin homo-epitaxial LaAlO.sub.3 interlayer between a LaAlO.sub.3 substrate and a YBCO film.